A machining method in which, in order to assemble a root portion of a turbine rotor blade to an outer peripheral groove of a turbine disk, an arc-shaped root groove of the root portion of the turbine rotor blade is machined is known from Patent Publication 1 below. In the method disclosed in Patent Publication 1, the surface of a roughly machined arc-shaped root groove, which is to be finish machined by cutting, is divided into four, the finish machining by cutting is carried out by reciprocating a cutting tool twice, and by carrying out the finish machining in different incising directions from each other in four steps contained in the two reciprocations of the cutting tool, cutting vibration is suppressed, thus improving the machining precision and the machining speed.
Furthermore, a method of milling a groove having a Christmas tree-shaped cross-section for seating a turbine blade leg on the outer periphery of a turbine shaft is known from Patent Publication 2 below. In the method disclosed in Patent Publication 2, a groove is milled so as to have as a preliminary shape a Christmas tree-shaped cross-section, which narrows stepwise by carrying out three roughly machining steps, and the groove having a Christmas tree-shaped cross-section is finish machined by passing a tool once in a finish machining step.
Furthermore, a method of machining in a turbine rotor an arc-shaped rotor groove having a Christmas tree-shaped cross-section, and a counterbore for carrying out the machining are known from Patent Publication 3. In the method disclosed in Patent Publication 3, an arc-shaped rotor groove is roughly machined in a turbine rotor, and the rotor groove is then finish machined by linear sideways movement of a hanging bell-shaped counterbore.
Moreover, a blade groove-machining cutter for machining a blade groove for mounting a turbine blade on the outer periphery of a rotor is known from Patent Publication 4. The cutter disclosed in Patent Publication 4 has an edge portion formed along a plurality of steps corresponding to a Christmas tree-shaped blade groove, and the rake angle of the edge portion is increased stepwise toward a front step (forward end portion) and a rear step (root portion).    Patent Publication 1: Japanese Patent Application Laid-open No. 6-270006    Patent Publication 2: Japanese Patent Publication (PCT) No. 2004-507369    Patent Publication 3: Japanese Patent Application Laid-open No. 9-234617    Patent Publication 4: Japanese Patent No. 3364168